1. Field of the Invention
The present invention relates to a method of suppressing beam position drift. The present invention also relates to a method of suppressing beam dimension drift. Furthermore, the present invention relates to a charged-particle beam lithography system. More particularly, the present invention relates to a method of suppressing beam position drift in such a way that the drift converges rapidly. In addition, the present invention relates to a method of suppressing beam dimension drift in such a way that the drift converges rapidly. Further, the present invention relates to a charged-particle beam lithography system in which such methods can be implemented.
2. Description of Related Art
An example of a variable-shaped electron beam lithography system generally used today is shown in FIG. 1. As shown in the figure, an electron beam 1 is made to hit a shaping apertured plate 3 via an illumination lens 2. An image 5 of a light source 4 is formed under the apertured plate 3. Then, an image of the apertured plate 3 is projected onto a second shaping apertured plate 7 via a shaping lens 6. A geometrical figure determined by overlap between the image of the shaping apertured plate 3 and the shaping apertured plate 7 is projected onto a material 10 via a demagnification lens 8 and an objective lens 9. A resist (photosensitive material) is applied on the material 10.
In this way, the resist is exposed. The shape and size of the projected figure can be varied by moving the image of the shaping apertured plate 3 relative to the shaping apertured plate 7 by a deflector 11. The position of the projected figure can be varied by another deflector 12. A pattern can be written on the material 10 by moving the projected figure into a desired position within the plane of the material 10 by the deflector 12.
A pattern can be written beyond a deflection field defined by the deflector 12 by driving the material stage to move the material surface. Electrostatic deflectors are used as the deflectors 11 and 12 to provide high deflection speeds.
As semiconductor devices have been fabricated to achieve ever larger scales of integration, i.e., finer patterns have been written, the required lithographic accuracies, including lithographic dimensional accuracy, and lithographic positional accuracy, have been made more stringent year by year. Recently, lithographic accuracy on the nanometer order has been required.
To improve the lithographic accuracy, it is important to stabilize the path of the electron beam 1. Variations in the path of the beam 1 cause drift in beam dimensions (i.e., cross-sectional dimensions of the beam on the material 10) and in beam position (i.e., the position at which the beam impinges on the material 10). Variations in the beam path are caused for the following reasons.
First, scattering electrons arising from the electron beam 1 and secondary electrons produce electrical charging in the electron optical column, giving rise to an electric field. This, in turn, deflects the beam 1. Because the amount of electric charge varies with time, the amount of deflection also varies with time. Alternatively, the electron beam 1 impinges on the shaping apertured plates 3 and 7, producing heat. This deforms the plates 3 and 7, shifting the positions of the apertures.
The amount of drift in beam dimensions is reduced as the beam dimensions are reduced, for example, by a factor of 10 by the demagnification lens 8. Variations in the beam path produced downstream of the demagnification lens 8 appear as beam position drift rather than as beam dimension drift.
For these reasons, in electron beam lithography systems typified by the optical system of FIG. 1, the amount of beam position drift tends to be greater than the amount of beam dimension drift in theory.
Drifts in beam dimensions and beam position due to variations in the beam path are compensated for by calibrating the system before or during a lithographic operation. The calibration is achieved by measuring and correcting the beam dimensions and position.
The amounts of drifts in beam dimensions and position are measured by detecting a backscattered electron signal and a transmission electron signal which are obtained when marks formed on the material 10 or on the material-moving stage on which the material is placed are scanned with a beam. The marks are used to measure the beam dimensions and position. The beam dimensions and position are corrected by operating the deflectors 11 and 12 by amounts corresponding to the measured dimensional deviations and positional deviation or by shifting the position of the material 10 by means of the material-moving stage.
It is known that drift in beam position increases immediately after the start of a lithographic operation. Even where the system is calibrated periodically during the lithographic operation, if the drift increases, the lithographic accuracy deteriorates.
In an attempt to solve this problem, it is customary to set a wait time prior to a lithographic operation to permit drift in beam position to converge to some extent. During the wait time, the beam current is made closer to the beam current used during the lithography. Writing is done on a region located outside the material 10. In this way, the beam position drift is made to converge.
It is considered that convergence of beam position drift depends on the balance between electrical charging and discharging or between generation and dissipation of heat. However, the time taken for the beam position drift to converge is as long as tens of minutes to several hours. Increasing the wait time is undesirable from the viewpoint of lithography throughput.
Beam position drift is also problematic during calibration of the system. Especially, when deflection field distortion is measured, it takes a long time to measure the beam position. That is, the beam is deflected across a number of points within the deflection field (e.g., 10×10=points). Therefore, beam position drift is increased accordingly during the measurement.
If beam position drift occurs, the drift will settle down after repetitive measurements. Consequently, it is not always necessary to set a wait time in advance when the system is calibrated. However, if the measurement of deflection field distortion is repeated until the beam position drift converges, the measurement time is eventually prolonged greatly.